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I was recently traveling in a remote mountain region and hiking alongside a very small river. What caught my attention was the fish in the small ponds under the waterfalls.

These ponds were just a couple of square meters in size and each one housed between 5 and 10 fish.

The course (do you mean flow?) of the river combined with the waterfalls mean it was basically out of question that they could swim / jump upstream.

In such small mountainous rivers, how do fish remain in the upper parts over the course of hundreds of years. I mean the size of the ponds and the amount of fish did not look to me like it could be sustainable for a population. And there should be the occasional one going down.

Are there examples or situations where the upstream population becomes extinct?

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  • $\begingroup$ Welcome to Biology.SE! I edited your post, but please check to make sure I haven't changed your meaning. In your third paragraph you say that "... course of the river combined with ..." — did you mean flow. If not, can you clarify what it is about the river that makes you think it was impossible for fish to migrate upstream? Thanks! 😊 $\endgroup$ – tyersome Aug 13 '20 at 1:02
  • $\begingroup$ @tyersome "Course of a river" is a common term. It refers to the path the river takes, including the path traveled across the surface of the earth, elevation changes, flow changes as a river goes through shallow/wide or narrow/deep sections, rapids, etc. $\endgroup$ – Bryan Krause Aug 13 '20 at 2:19
  • $\begingroup$ things to consider 1. some fish are very good at getting upstream and some can even climb the rock behind a waterfall 2 a rivers course now is not what it has always been, just because there is a waterfall now does not mean there was always one. $\endgroup$ – John Aug 13 '20 at 3:37
  • $\begingroup$ @BryanKrause — I am familiar with that usage, but in the context of that sentence (using "combined with") it appeared to me that the OP might mean something different, which is why I requested clarification ... $\endgroup$ – tyersome Aug 13 '20 at 22:57
  • $\begingroup$ Sorry, my bad, I'm not a native English speaker :) @Bryan Krause is right $\endgroup$ – KarlKarlsom Aug 14 '20 at 0:07

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